Monolithic integration of an optical detector using silicon electronic technique is quite attractive from the viewpoint of cost and yield. A monolithically integrated silicon photodetector integrated on a chip together with a CMOS circuit, i.e., a silicon photodiode, is an attractive alternative for a hybrid photodetector, such as an InGaAs photodiode connected to a CMOS circuit or to a GaAs circuit.
A monolithically integrated photodetector can be manufactured using a common silicon process and is expected to be manufactured at lower cost than a hybrid design.
Photodiodes are often used as a means for converting an optical signal to an electric signal at high speed. One typical example thereof is a pin-type photodiode. The pin-type photodiode has a structure in which an i layer of an intrinsic semiconductor is interposed between a p layer of a p-type semiconductor and an n layer of a n-type semiconductor. When a reverse bias voltage is applied to the photodiode from a bias power supply, substantially the entire region of the high resistant i layer becomes a depletion layer devoid of charge carriers. Photons of incident light are mainly absorbed by the i layer and produce electron/hole pairs. The electrons and holes that have been produced drift through the depletion layer in directions opposite to each other under the reverse bias voltage, producing an electric current, and the current is then detected as a signal voltage by a load resistor. Main factors that limit response speed of the photoelectric conversion include a time constant of the circuit, which is defined as the product of the load resistance and electric capacitance created by the depletion layer, and carrier running time required for electrons and holes to pass through the depletion layer.
One type of photodiodes having short carrier running time is the Schottky type photodiode. The Schottky type photodiode has a structure in which a translucent metal film is in contact with an n layer or an n-layer of a semiconductor. A Schottky barrier is formed near the interface on which the n layer or the n-layer and the translucent metal film are in contact with each other. The area near the Schottky barrier becomes a depletion layer because electrons scatter from the translucent metal film into the n layer or the n-layer. When incident light is radiated in this state, electrons are generated in the n layer or in the n-layer and drift through the depletion layer under the reverse bias voltage. Furthermore, the Schottky type photodiode can effectively use optical absorption on the element surface layer. While the pin-type photodiode requires a sufficient thickness of the i layer or the depletion layer in order to absorb photons the Schottky photodiode requires a thinner depletion layer, and thereby provides a reduction in carrier running time. Furthermore, studies have been made about a lateral electrode structure in order to reduce the thickness of the depletion layer of a pin-type photodiode and thereby to reduce a gap between electrodes, as described in Non-Patent Document 1. However, it is difficult for this structure to achieve high sensitivity because of the poor optical absorption efficiency on the surface layer of semiconductor, although it provides an enhanced response.
On the other hand, reducing the value of load resistance to shorten the time constant of the circuit causes a decrease in the voltage of a reproduced signal that is detected. Therefore, reducing the electric capacitance of the depletion layer is necessary to improve the SN ratio of a reproduced signal and to reduce read errors. Especially, since reducing the thickness of the depletion layer to shorten the carrier running time causes an increase in electric capacitance, it is necessary to reduce the area of the depletion layer or the Schottky junction in order to enhance response. However, reducing the junction area is problematic because it causes a reduction in utilization efficiency of signal light, which results in degradation of the SN ratio of the reproduced signal.
To cope with these problems, various attempts have been made along with the recent development of technology that aim at achieving high-speed response and downsizing of photoelectric conversion devices of this type, compared to conventional devices, by using metal surface plasmons or a photonic crystalline structure.
For example, Patent Document 1 describes an optical detector that uses a metal/semiconductor/metal (MSM) device having two electrodes disposed on one surface of semiconductor. The MSM-type optical detector is a kind of Schottky photodiode that generally has a Schottky barrier near the two electrodes. Light which passes through the electrode surface is partially absorbed in the semiconductor layer, producing photocarriers. The MSM-type optical detector is disadvantageous in that increasing the thickness of a semiconductor for the purpose of increasing quantum efficiency causes an increase in the propagation distance of photocarriers and resultantly causes a slow response. To prevent the slow response, the optical detector described in Patent Document 1 has metal electrodes that are disposed along periodically arranged convex and concave parts. Incident light can be efficiently coupled with surface plasmon of the metal electrodes and can propagate into the optical detector.
Patent Document 2 describes a method for manufacturing an MSM-type light receiving element by forming a metal film on a semiconductor, and by oxidizing part of the metal film to form a light transmitting insulator pattern.
Patent Document 3 describes enhancing the response of an MSM-type light receiving element by reducing the width of a light transmitting insulator pattern to the wavelength or less in order to utilize an optical near field that is generated from both ends of a metal film located on both sides of the light transmitting insulator pattern.
Patent Document 4 describes a photoelectron coupler in which positive electrodes and negative electrodes are arranged on a semiconductor at a regular interval such that the positive and negative electrodes are nested in each other. This document describes a technique for coupling incident light, transmitted light, reflected light and surface plasmon polariton etc. with each other, through resonance, by using such a device structure. This document describes that a MSM-type light receiving element disclosed herein that uses a photoelectron coupler technique enables intensification of photocarriers generated by incident light by coupling between incident light and surface plasmon. However, these light receiving elements are problematic because reducing the radiation area of incident light in order to reduce the electric capacitance of the depletion layer leads to a reduction in the intensity of the detected signal and deterioration of the SN ratio.
Patent Document 5 describes a photovoltaic device that uses solar energy. In this photovoltaic device, one of two electrodes that interpose a plurality of spherical semiconductors having a pn junction is provided with periodically arranged openings or concave parts. The device uses resonance between surface plasmon on the electrodes having a periodic pattern and incident light. However, this technique does not describe reducing the thickness and the area of a depletion layer in order to enhance response of photoelectric conversion.
Furthermore, Patent Document 6 describes an optical transmission apparatus that is capable of intensifying propagating light by disposing a periodic groove array around an opening compared to a case having no periodic groove array, even if only a single opening is provided. However, it is known that the total energy of transmitted light is attenuated compared to the energy of incident light. According to a document (Tineke Thio, H. J. Lezec, T. W. Ebbesen, K. M. Pellerin, G. D. Lewen, A. Nahata, R. A. Linke, “Giant optical transmission of sub-wavelength apertures: physics and applications,” Nanotechnology, vol. 13, pp. 429-432, FIG. 4), the total energy of light that is transmitted through an opening having a diameter of 40% or less of the wavelength is attenuated to 1% or less of incident light energy. Therefore, a high SN ratio can not be obtained even if propagating light from this optical transmission apparatus is radiated onto a light receiving element.
Furthermore, Patent Document 7 describes an MSM light receiving element structure that has a light absorbing layer having a multilayered structure to form a photonic band. The physical absorption thickness is reduced by decreasing the group velocity of light that is absorbed and transmitted, and thereby light receiving efficiency is increased. However, this structure does not realize a reduction in the junction area of the MSM junction and a resultant reduction in the element capacitance.    Non-Patent Document 1: S. J. KOESTER, G. Dehlinger, J. D. Schaub, J. O. Chu, Q. C. Ouyang, and A. Grill, “Germanium-on-Insulator Photodetectors”, 2nd International Conference on Group 4 Photonics, FB1 2005, (p. 172, FIG. 3)    Patent Document 1: Japanese Patent Application Laid-Open No. 108376/84 (p 4-16, FIGS. 1-3)    Patent Document 2: Japanese Patent No. 2666888 (p 3-4, FIG. 2)    Patent Document 3: Japanese Patent No. 2705757 (p 6, FIG. 1)    Patent Document 4: Japanese Patent Application Laid-Open No. 509806/98 (p 26-33, FIG. 1)    Patent Document 5: Japanese Patent Application Laid-Open No. 2002-76410 (p 6-9, FIG. 1)    Patent Document 6: Japanese Patent Application Laid-Open No. 2000-171763 (p 7-10, FIG. 10, FIG. 17)    Patent Document 7: Japanese Patent Application Laid-Open No. 2005-150291 (p 5, FIG. 1)
The MSM photodiode provides planarity and compatibility with a silicon LSI. However, the optical detector using Si or SiGe generally shows a slow response due to a long carrier life time (˜1 to 10 μs) and a low optical absorption rate (˜10 to 100/cm). Furthermore, in the case of a compound semiconductor, a Schottky barrier type photodiode provides a high-speed response, but is problematic because the metal electrodes reduce the effective light receiving area and reduce sensitivity. On the other hand, in the case of a pin-type photodiode, a lateral electrode structure is proposed to reduce the thickness of a depletion layer. This structure can reduce the distance between the electrodes to achieve a high-speed response, but is problematic because it is difficult to achieve high sensitivity.
Enhancement of the response of the photodiode requires a reduction in the thickness of the light absorbing layer that shortens carrier running time and a reduction in the light receiving area, i.e., junction capacity, that reduces the time constant of the circuit. Thus, there generally exists a trade-off relationship between light receiving sensitivity and high-speed response.
Furthermore, a lift-off process using a resist mask is generally used to form electrodes of a Schottky photodiode. However, this process disadvantageously tends to cause a yield reduction in the device manufacturing process, as well as electrical short circuits between electrodes generated by lift-off residues.
The present invention aims at providing a device structure of a photodiode that achieves both light receiving sensitivity and a high-speed response. The present invention also aims at providing a photodiode that achieves a high degree of integration and low power consumption by realizing a light absorbing layer that has a smaller volume than that of related arts by a factor of one hundredth or less. Furthermore, the present invention aims at providing a method for manufacturing a photodiode that is suitable for mass production and for achieving high degree of integration.